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[37.188.130.62]) by smtp.gmail.com with ESMTPSA id z16sm1124064wrl.0.2020.04.20.05.48.00 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 20 Apr 2020 05:48:00 -0700 (PDT) Date: Mon, 20 Apr 2020 14:47:59 +0200 From: Michal Hocko To: Linus Torvalds Cc: Peter Xu , Linux Kernel Mailing List , Linux-MM , Andrew Morton , syzbot+693dc11fcb53120b5559@syzkaller.appspotmail.com Subject: Re: [PATCH 1/2] mm/mempolicy: Allow lookup_node() to handle fatal signal Message-ID: <20200420124759.GO27314@dhcp22.suse.cz> References: <20200408014010.80428-1-peterx@redhat.com> <20200408014010.80428-2-peterx@redhat.com> <20200409070253.GB18386@dhcp22.suse.cz> <20200414110429.GF4629@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200414110429.GF4629@dhcp22.suse.cz> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Any opinion on this Linus? Should I just repost the patch? On Tue 14-04-20 13:04:32, Michal Hocko wrote: > On Thu 09-04-20 09:42:20, Linus Torvalds wrote: > > On Thu, Apr 9, 2020 at 12:03 AM Michal Hocko wrote: > > > > > > This patch however doesn't go all the way to revert it because 0 return > > > value is impossible. > > > > I'm not convinced it's impossible. > > __get_user_pages is documented as > * -- If nr_pages is 0, returns 0. > * -- If nr_pages is >0, but no pages were pinned, returns -errno. > * -- If nr_pages is >0, and some pages were pinned, returns the number of > * pages pinned. Again, this may be less than nr_pages. > > but let me double check the actual code... There seem to be only one > exception the above rule AFAICS. faultin_page returning EBUSY will > be overriden to either 0 for the first page or return the number of > already pinned pages. So nr_pages > 0 && ret = 0 is indeed possible > from __get_user_pages :/ That will be the case only for VM_FAULT_RETRY, > thoug. > > Now __get_user_pages_locked behaves differently. It keeps retrying the > fault until it succeeds unless FOLL_NOWAIT is specified. Then it would > return 0. Why we need to return 0 is not really clear to me but it > seem to be a long term behavior. I believe we need to document it. > > > And if it is, then the current code is harmless. > > Yes from the above it seems that the check is indeed harmless becasue > this path doesn't use FOLL_NOWAIT and so it will never see 0 return. > I find a reference to EINTR confusing so I would still love to change > that. > > > Now, I do agree that we probably should go through and clarify the > > whole range of different get_user_pages() cases of returning zero (or > > not doing so), but right now it's so confusing that I'd prefer to keep > > that (possibly unnecessary) belt-and-suspenders check for zero in > > there. > > > > If/when somebody actually does a real audit and the result is "these > > functions cannot return zero" and it's documented, then we can remove > > those checks. > > Would you mind this patch instead? > > commit bc6c0fa7c7fb5eb54963dca65ae4a62ba04c9efa > Author: Michal Hocko > Date: Thu Apr 9 08:26:57 2020 +0200 > > mm, mempolicy: fix up gup usage in lookup_node > > ba841078cd05 ("mm/mempolicy: Allow lookup_node() to handle fatal signal") has > added a special casing for 0 return value because that was a possible > gup return value when interrupted by fatal signal. This has been fixed > by ae46d2aa6a7f ("mm/gup: Let __get_user_pages_locked() return -EINTR > for fatal signal") in the mean time so ba841078cd05 can be reverted. > > This patch however doesn't go all the way to revert it because the check > for 0 is wrong and confusing here. Firstly it is inherently unsafe to > access the page when get_user_pages_locked returns 0 (aka no page > returned). > Fortunatelly this will not happen because get_user_pages_locked will not > return 0 when nr_pages > 0 unless FOLL_NOWAIT is specified which is not > the case here. Document this potential error code in gup code while we > are at it. > > Signed-off-by: Michal Hocko > > diff --git a/mm/gup.c b/mm/gup.c > index 50681f0286de..a8575b880baf 100644 > --- a/mm/gup.c > +++ b/mm/gup.c > @@ -980,6 +980,7 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags) > * -- If nr_pages is >0, but no pages were pinned, returns -errno. > * -- If nr_pages is >0, and some pages were pinned, returns the number of > * pages pinned. Again, this may be less than nr_pages. > + * -- 0 return value is possible when the fault would need to be retried. > * > * The caller is responsible for releasing returned @pages, via put_page(). > * > @@ -1247,6 +1248,10 @@ int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm, > } > EXPORT_SYMBOL_GPL(fixup_user_fault); > > +/* > + * Please note that this function, unlike __get_user_pages will not > + * return 0 for nr_pages > 0 without FOLL_NOWAIT > + */ > static __always_inline long __get_user_pages_locked(struct task_struct *tsk, > struct mm_struct *mm, > unsigned long start, > diff --git a/mm/mempolicy.c b/mm/mempolicy.c > index 48ba9729062e..1965e2681877 100644 > --- a/mm/mempolicy.c > +++ b/mm/mempolicy.c > @@ -927,10 +927,7 @@ static int lookup_node(struct mm_struct *mm, unsigned long addr) > > int locked = 1; > err = get_user_pages_locked(addr & PAGE_MASK, 1, 0, &p, &locked); > - if (err == 0) { > - /* E.g. GUP interrupted by fatal signal */ > - err = -EFAULT; > - } else if (err > 0) { > + if (err > 0) { > err = page_to_nid(p); > put_page(p); > } > -- > Michal Hocko > SUSE Labs -- Michal Hocko SUSE Labs